High-temperature ovens, called reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A circular wafer or substrate, typically made of silicon, is placed on a wafer support called a susceptor. Both the wafer and susceptor are enclosed in a quartz chamber and heated to 600.degree. C. (1112.degree. F.) or higher, typically by a plurality of radiant lamps placed around the quartz chamber. A reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes in other equipment, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits.
If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons including its greater precision as opposed to processing batches of wafers at the same time. Although single-wafer processing by itself provides advantages over batch processing, control of the process parameters remains critical and is perhaps more so because of the increased cost of the larger wafers. One especially important element is the reduction of particulate matter within the reaction chamber which might contaminate the purity of the resulting semiconductor wafers.
Another problem in wafer manufacturing is the contamination of the reaction chamber with water molecules. That is, some small concentration of airborne water particles inevitably migrates into the reaction chamber along with the wafer. This is because the wafers are initially loaded from an external location, and is very difficult to purge completely the moisture in the air. Indeed, some small number of water molecules may adhere to the surface of each wafer. Water in the reaction chamber can create nonuniformities in the subsequently deposited chemical layers.
In many continuous processing systems, as well as multi-chamber systems, the wafers are loaded through an input port and then passed through one or more intermediate chambers before entering the reaction chamber. One such multiple chamber system is disclosed in U.S. Pat. No. 5,156,521, issued to Crabb, et al. In the Crabb device, a magazine of wafers is placed within an input chamber through a door. The door is then closed, and the chamber is purged with nitrogen. An elevator lowers the magazine through an aperture in the floor of the input chamber and into registration with an entry port into a handling chamber. A robot arm having a pick-up wand on one end then removes wafers from the magazine and transfers them to one of the reaction chambers surrounding the handling chamber. Isolation valves are provided between the handling chamber and the various reaction chambers. The environment within the handling chamber is thus sealed from that within the reaction chambers. Furthermore, the pick-up wand operates on a Bernoulli principal in that gas flows downward from the pick-up wand to lift the individual wafers by creating a reduced pressure on top of the wafers due to the gas velocity over the top surface.
Because many thermal CVD processes involve extremely high temperatures within the reaction chamber to catalyze the chemical deposition process, the throughput of the system is slowed as the hot wafers are removed from the reactor and deposited in a waiting unload magazine, for example. Bernoulli wands are beneficial in this respect because they need not directly contact the wafers, and thus can lift them when they are hot. There have been a number of attempts to design secondary cooling chambers within which the wafers are placed to cool rapidly and speed the throughput of the system. However, the separate chambers add expense to the system.
Currently, there is a need for a processing system which more effectively prevents unwanted gases from entering the reaction chamber.